The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Typically, in nuclear boiling water reactors (BWRs), the amount of reactivity of the core is controlled by positioning of control rods and adjusting the amount of liquid coolant recirculation flow through the reactor core. For example, the power can be adjusted 30%-40% simply by changing the amount of recirculation flow. Generally, typical BWRs are set to operate at a relatively low power level, i.e., where the fuel bundles are operating at fairly low powers, by moving control rods and then the recirculation flow can be increased to obtain 100% power without moving any control rods. By utilizing the recirculation flow to control power output, the fuel rods and fuel pellets change power levels at a relatively slow, uniform rate, thereby avoiding pellet clad interaction (PCI) and damage to the fuel rod cladding.
However, natural circulation boiling water reactors (NCBWRs) do not include recirculation pumps, but instead employ natural circulation. Therefore, recirculation flow cannot be utilized to control the power output of NCBWRs. At least some known NCBWRs manipulate the control rods to control the power level of fuel bundles, which causes the power level to change at fairly rapid, non-uniform rates. Increasing the power too rapidly as the control rods are withdrawn can cause significant damage to the fuel cladding.